The present invention relates to a magnetic recording system, and more particularly, is directed to a magnetic recording system having a recording density higher than 2 gigabits per one square inches and a thin film magnetic recording medium having a low media noise but having a sufficient stability against a thermal fluctuation in order to become able to realize such a magnetic recording system.
At present, there is an increasing demand that a magnetic recording system becomes larger in storage capacity. As a magnetic head, there has hitherto been used an inductive head which makes effective use of a phenomenon in which a voltage is changed as a magnetic flux is changed with a time. According to the above-mentioned inductive head, a single magnetic head is able to both write and read information in and from a magnetic recording medium.
Recently, there is rapidly spread a combination type magnetic head comprising a record head and a write head and in which a magnetoresistive head of a higher efficiency is used as the read head. The magnetoresistive head makes effective use of a phenomenon in which an electric resistance of a head element is chanced in accordance with a change of a leakage flux from a magnetic recording medium. On the other hand, there has hitherto been developed a head of a much higher efficiency which makes effective use of a considerably large magnetoresistive change (i.e. giant magnetoresistive effect or spin valve effect) generated in a magnetic layer of the type such that a plurality of magnetic layers are laminated with each other through a non-magnetic layer. This giant magnetoresistive effect is such one that relative directions of magnetization of a plurality of magnetic layers laminated with each other through the non-magnetic layer are changed by a leakage field from a magnetic recording medium, thereby resulting in an electric resistance being changed.
To realize a high recording density, it becomes necessary to further reduce a media noise. A study of measured results obtained by a computer simulation and measured results obtained by experiments reveals that, in order to reduce a media noise, it is effective to reduce an exchange interaction between magnetic crystal grains or to reduce a grain size of a magnetic crystal grain (J. Appl. Phys., Vol. 63(8), 3248 (1988), J. Appl. Phys., Vol. 79(8), 5339 (1966)). Specifically, as a method of reducing an exchange interaction, there are mainly enumerated a method of increasing a Cr content in a magnetic layer, a method of increasing a spatial separation of magnetic crystal grains and the like. When the Cr content of the magnetic layer is increased, a larger amount of Cr may be segregated into the crystal grains, thereby decreasing the exchange interaction between the magnetic crystal grains. However, at the same time, the Cr content in the magnetic crystal grains also is increased, thereby resulting in a saturated magnetic flux density being lowered. Therefore, the film thickness of the magnetic layer should be increased in order to maintain a value of Brxc3x97t which is a product of a residual magnetic flux density Br and a film thickness t of a magnetic layer. However, since the crystal grain increases its size as the film thickness of the magnetic layer increases thereby to cause a media noise to increase, this method has a limit. Moreover, in order to increase the separation of the magnetic crystal grains, it is necessary to reduce a size of crystal grains of each underlayer having an acicular structure. To this end, a film thickness of an underlayer has to be increased. Also in this case, it is unavoidable that a size of a magnetic crystal grain formed on the underlayer is increased. Thus, this method also has a limit.
Further, Japanese laid-open patent application No. 7-311929 describes a method in which oxide such as SiO2 is added to a magnetic layer, the resultant product is segregated into the crystal grains thereby to reduce an exchange interaction between crystal grains and in which, at the same time, a grain size of a crystal grain may be decreased by suppressing a crystal growth. However, because the addition of the insulating material causes a resistance value of a target to increase considerably, it is unavoidable that a film is deposited by an RF (radio frequency) sputtering. As compared with a DC (direct current) sputtering, the RF sputtering is inferior to the DC sputtering from a standpoint of a manufacturing cost and a stability or the like, and hence is not suitable for a mass-production. Moreover, since a crystallographic orientation of a magnetic recording medium manufactured by the RF sputtering is difficult to control, there arises such a problem that it is very difficult to obtain a high coercivity and a coercivity squareness.
Furthermore, since the magnetic crystal grain of which the grain size is reduced is much more strongly affected by the influence of the thermal fluctuation, a stability of a recorded magnetization is lowered considerably. As a consequence, a ratio in which an inversion of magnetization occurs increases with a time so that it becomes unable to maintain a sufficiently high reliability when data is saved during a long period of time. Further, since the crystal grain the grain size of which is reduced is much more strongly affected by a magnetostatic interaction from the adjacent crystal grain, there is unavoidably caused the increase of a media noise. Accordingly, it is desirable that the crystal grain should keep a certain large size.
On the other hand, heretofore, there have been reported a large number of experiments in which a magnetic layer is formed as a bilayer or a multilayer. Japanese laid-open patent application No. 8-147660, for example, describes a magnetic recording medium having a high coercivity and a low media noise. This magnetic recording medium comprises two magnetic layers in which a first magnetic layer is formed of a CoCrTa alloy layer and a second magnetic layer is formed of a CoCrPtTa alloy layer. However, according to this previously-proposed magnetic recording medium, since an influence exerted by excessively small crystal grains existing at the initial crystal growth portion of the magnetic layer cannot be eliminated, there cannot be expected an effect for suppressing a decay of a read signal due to a thermal fluctuation. The decay of the read signal due to the thermal fluctuation becomes a serious problem particularly when the magnetic layer is reduced in thickness. Also, Japanese laid-open patent application No. 8-77544, for example, describes a magnetic recording medium having a high coercivity in which a magnetic layer has a bilayer laminated structure comprising a soft magnetic layer and a hard magnetic layer. However, if the magnetic layer contains the soft magnetic layer, there is then the large possibility that a media noise will increase due to a strong exchange interaction. Further, such a magnetic recording medium should not be preferable because it is easily affected by an external magnetic field. Furthermore, Japanese laid-open patent application No. 6-243454, Japanese laid-open patent application No. 6-342511 and Japanese laid-open patent application No. 6-349047, for example, describe magnetic recording media in which a magnetic layer is divided by a non-magnetic intermediate layer such as a Cr non-magnetic layer. Since the non-magnetic intermediate layer weakens the exchange interaction between the divided magnetic layers, the size of the magnetization inversion is reduced, and as a result, media noise can be reduced. However, as is described in IEEE TRANSACTIONS ON MAGNETICS, Vol. 30, pp. 4230-4232 (published in 1994), since a magnetostatic interaction acts on the divided magnetic layers, if the size of the magnetization inversion is reduced to such an extent that it is affected by a thermal fluctuation, there is then the possibility that a magnetization will be canceled out by this negative interaction.
As described above, in the magnetic recording media using the multilayer magnetic layer according to the examples of the related art, although magnetic properties can be improved to some extent but such improvements are not sufficient. Besides, it cannot be expected to suppress the influence of the thermal fluctuation which becomes remarkable particularly when the magnetic layer is reduced in thickness. Therefore, the above-mentioned magnetic recording media according to the related art are not sufficient for realizing a high recording density which is greater than 2 gigabits per square inch.
As described above, in order to realize a high recording density, it becomes necessary to provide a magnetic recording medium which not only has a low media noise but also has a sufficiently high stability against a thermal fluctuation.
In view of the aforesaid aspect, it is an object of the present invention to provide a magnetic recording medium having not only a low media noise but also a sufficiently high stability against a thermal fluctuation by controlling an average grain size and a grain-size dispersion of a magnetic crystal so as to fall within a range of proper values.
It is another object of the present invention to provide a highly-reliable magnetic recording system having a high recording density higher than 2 gigabits per one square inches by a combination of this magnetic recording medium and a magnetic head of a high efficiency.
The above-mentioned objects can be achieved by a magnetic recording medium comprising a substrate and a magnetic layer deposited on the substrate through a single underlayer or a multilayer underlayer and in which an average grain size  less than d greater than  of a crystal grain in the magnetic layer is selected to be less than 16 nm and a grain-size dispersion xcex94d/ less than d greater than  (hereinafter referred to as xe2x80x9cnormalized grain-size dispersionxe2x80x9d) normalized by the average grain size is selected to be less than 0.5.
The manner in which the crystal grain size and the normalized grain-size dispersion are calculated will be described below. Initially, a magnetic recording medium is polished to a thickness of several 10 s of microns, and a film thickness of a magnetic layer is reduced to about 10 nm by ion thinning. Then, a lattice image is observed in a high resolution mode under a transmission electron microscope (TEM), and there is obtained a plane TEM image (lattice image) of about xc3x971000000 to xc3x972000000 developed on a printing paper. Further, this lattice image is read in by an image scanner, the lattice image is displayed on a screen of a personal computer, and lines are drawn along a grain boundary, thereby resulting in a crystal grain boundary network being manufactured. At that time, the grain boundary is used as a portion in which a lattice stripe is changed (crossed). When there is observed a structure in which a plurality of magnetic crystal grains with different crystal orientations are grown on the same underlayer crystal grain, i.e. bi-crystal structure, crystal grains (sub grains) having the same crystal orientation are counted as one crystal grain. FIG. 1 shows an example of the crystal grain boundary network thus obtained. An area of each crystal grain encircled by this grain boundary network was calculated by using a commercially-available particle analysis software, and a diameter of a true circle having the same area as that of the above-mentioned crystal grain was set to the grain size of each crystal grain. Crystal grain sizes of 100 to 300 crystal grains were calculated by the above-mentioned method. A value which results from normalizing a sum total of areas of some crystal grains the grain sizes of which are less than the observed crystal grains by the total areas of all measured crystal grains was defined as an accumulated area fraction. FIG. 2(a) shows an example of a relationship (hereinafter referred to as xe2x80x9caccumulated area fraction curvexe2x80x9d) between the magnetic crystal grain size of the medium according to the present invention and the accumulated area fraction. FIG. 2-(b) shows an example of a histogram of a grain-size frequency. Herein, the average grain size  less than d greater than  is defined as a grain size obtained when the accumulated area fraction becomes 50%, and the grain-size dispersion width is defined as a difference between a grain size obtained when the accumulated area fraction becomes 75% and a grain size obtained when the accumulated area fraction becomes 25%. Further, a ratio xcex94d/ less than d greater than  between the grain-size dispersion width xcex94d and the above-mentioned average grain size  less than d greater than  is defined as a normalized grain-size dispersion. Incidentally, when a magnetic particle is amorphous, a grain size is analyzed based on a bright visual field image of a magnetic layer by a similar method.
When the average grain size becomes larger than 16 nm, if information is recorded with a high linear recording density greater than 200 kFCI, then a disturbance (irregularity) of a magnetization transition area becomes large and a media noise increases, which should not be preferable. Also, if the normalized grain-size dispersion exceeds 0.5, then the number of extremely-small crystal grains increases and these extremely-small crystal grains are strongly affected by the influence of the thermal fluctuation with the result that a decay of recorded magnetization becomes considerable, which should not be preferable. Furthermore, at that time, it is not preferable that the media size also increases. As described above, not only to reduce the grain size of the magnetic crystal grain simply but also to exclude extremely-small crystal grains, which are easily affected by the thermal fluctuation, by making the grain sizes of the magnetic crystal grains become uniform, are very important factors for reducing the media noise and for maintaining the stability against the thermal fluctuation. If the normalized grain-size dispersion is selected to be less than 0.4 as seen in the inventive example 2 which will be described later on, then the media noise can be further decreased and the thermal stability can be further improved, which are more preferable. Furthermore, the control of the magnetic crystal grain size becomes an extremely-effective noise reducing means for a magnetic recording medium using a glass substrate which is difficult to reduce a media noise as compared with a related-art magnetic recording medium using, in particular, an NiP/Al substrate.
Although a magnetic layer may be made of alloys using Co such as CoCrPt, CoCrPtTa, CoCrPtTi, CoCrTa or CoNiCr as principal components and which also contains Cr, Co alloy containing Pt should preferably be used in order to obtain a high coercivity. If a value Kuxc2x7v/kT which results from dividing a product of a magnetic anisotropy energy Ku of a magnetic crystal grain and a volume v by a product of a Boltzmann constant k and an absolute temperature T is selected to be greater than 60, then it becomes possible to reduce the influence of the thermal fluctuation. If the value of Kuxc2x7v/kT becomes less than 60, then there rapidly increases the probability that the inversion of the magnetization will occur due to the influence of the thermal fluctuation. As a result, the decay of the recorded magnetization becomes remarkable, which is not preferable. Further, particularly in a magnetic recording medium in which a grain size of a magnetic layer becomes less than 10 nm, the magnetic layer should preferably be made of a high Ku material such as alloys having a high Pt content in the magnetic layer or alloys containing rare-earth elements such as SmCo or FeSmN. The film thickness of the magnetic layer should preferably be less than twice the average grain size. If the film thickness of the magnetic layer exceeds twice the average grain size, then a shape magnetic anisotropy in the direction perpendicular to the film surface is increased so that a coercivity in the in-plane direction decreases, which should not be preferable.
If on the other hand the above-mentioned value of Kuxc2x7v/kT increases excessively, then a coercivity increases. As a result, there arises a problem that a sufficient overwrite property cannot be obtained during write/read operation. Having variously examined the medium structures which are able to provide an excellent overwrite property, the assignee of the present application discovered that such a medium structure is very effective in which a magnetic layer is formed as a bilayer structure wherein a Pt content in a magnetic layer (hereinafter referred to as xe2x80x9cfirst magnetic layerxe2x80x9d) adjoining an underlayer is made higher than a Pt content in a magnetic layer (hereinafter referred to as xe2x80x9csecond magnetic layerxe2x80x9d) deposited on the first magnetic layer. When the Pt content in the first magnetic layer is made high, there can be achieved the effect for suppressing excessively-small particles in the initial growth portion of the magnetic layer from being fluctuated thermally. When the Pt content in the second magnetic layer is made low, a value of a coercivity may be optimized. Further, if a Cr content in the first magnetic layer is made lower than a Cr content in the second magnetic layer thereby to leave a proper exchange interaction between the grains, then it is possible to improve the above-mentioned effect. Specifically, the excessively-small grains in the initial growth portion of the magnetic layer are magnetically combined to make a substantial magnetization inversion size become larger than the grain size, whereby the influence of the thermal fluctuation may be suppressed. Therefore, the thickness of the first magnetic layer should preferably be made approximately the same as that of the initial film deposition portion of the magnetic layer. Since the thickness of the initial film deposition portion is about 5 nm at the maximum although it is changed depending upon the combination of the underlayer and the magnetic layers and the film deposition process or the like, the film thickness of the first magnetic layer should preferably be made less than 5 nm. Furthermore, the total film thickness of the first magnetic layer and the second magnetic layer should preferably be made less than 15 nm from a standpoint of increasing a resolution of a magnetic recording medium.
With respect to the magnetic properties of the magnetic layer, if a coercivity measured when a magnetic field is applied to the recording direction is set to be greater than 2 kOe and the product Brxc3x97t of the residual magnetic flux density Br and the film thickness t is set to be greater than 40 Gaussxc2x7micron and smaller than 140 Gaussxc2x7micron, then excellent read/write characteristics should be obtained in a recording density region the recording density of which is higher than 2 gigabits per one square inches, which should be preferable. If a coercivity becomes smaller than 2 kOe, then the output at the high recording density (greater than 200 kFCI) becomes small, which should not be preferable. Also, if the product Brxc3x97t becomes larger than 140 Gaussxc2x7micron, then the resolution is lowered. If on the other hand the product Brxc3x97t becomes smaller than 40 Gaussxc2x7micron, then the read output becomes small, which should not be preferable.
Further, if a carbon layer having a thickness ranging from 5 nm to 20 nm is formed as a protective layer of a magnetic layer and a lubricant layer having a thickness ranging from 2 nm to 10 nm is made of perfluoroalkyl-polyether or the like having an adsorptive property, then there can be obtained a highly-reliable magnetic recording medium which is capable of executing a high density recording. Further, if a carbon film added with hydrogen, a film made of a compound such as silicon carbide, tungsten carbide, (Wxe2x80x94Mo)xe2x80x94C, (Zrxe2x80x94Nb)xe2x80x94N or a mixed film of these compounds and carbon is used as a protective layer, then a slide resistance and a corrosion resistance can be improved, which should be preferable. Furthermore, after these protective layers are formed, if the surface is made very slightly uneven by plasma etching with a particle mask or protrusions of different phases are produced on the surface of the protective layer by using targets of compounds and mixtures or the surface is made uneven by heat treatment, then a contact area in which the head and the magnetic recording medium are brought in contact with each other may be reduced. Hence, the problem that the head adheres to the surface of the medium upon CSS (contact start stop) operation can be avoided, which should be preferable.
In a magnetic recording system comprising the above-mentioned magnetic recording medium, a head access system for driving this magnetic recording medium in the recording direction, a magnetic head composed of a write unit and a read unit, a means for moving the magnetic head relative to the magnetic recording medium and a read/write signal processing means for writing a signal on the magnetic head and reading an output signal from the magnetic head, if the read unit of the magnetic head is formed of a magnetoresistive head, it is possible to obtain a sufficiently-high intensity of signal in a high recording density. Thus, there may be realized a highly-reliable magnetic recording system which may provide a recording density of higher than 2 gigabits per one square inches. Also, a spacing (shield spacing) between two shield layers which sandwich a magnetoresistive sensor unit of the magnetoresistive head used in the magnetic recording system according to the present invention should preferably be made less than 0.30 xcexcm. The reason for this is that, if the shield spacing becomes greater than 0.30 xcexcm, then a resolution is lowered, and a phase jitter of a signal increases. Furthermore, if the magnetoresistive head is comprised of a magnetoresistive sensor including a plurality of conductive magnetic layers which are able to generate a large change of a resistance as the magnetization directions thereof are relatively changed with an application of an external magnetic field and a conductive non-magnetic layer disposed between the conductive magnetic layers and a giant magnetoresistive effect or a spin valve effect is used, then an intensity of a signal can be further increased. Thus, it becomes possible to realize a highly-reliable magnetic recording system having a recording density higher than 4 gigabits per one square inches.